Ceramic composition for thermistor, thermistor element, and process for producing same

ABSTRACT

A ceramic composition for thermistor exhibiting stable resistance values over a wide temperature range and capable of being used for a prolonged time. The composition is represented by (M 1  M 2  O 3 ) 1-x  ·(M 1  AlO 3 ) x , wherein M 1  is an element selected from the elements of the group 3A of the International Periodic Table excluding La and M 2  is an element of the groups 4A, 5A, 6A, 7A and 8 of the International Periodic Table, and wherein the mixing ratio between an electrically conductive substance stable at elevated temperatures and an insulating substance stable at elevated temperatures, may be adjusted. A thermistor formed of the composition is superior in high-temperature stability and can be used over a broad temperature range because its resistance value can be selected to an optimum value.

FIELD OF THE INVENTION

This invention relates to a ceramic (or porcelain) composition for athermistor which is superior in high temperature stability.

BACKGROUND

As a thermistor material which may be employed at higher temperatures,(a) a material mainly composed of a corundum type crystal structure, asdisclosed for example in JP Patent KOKAI Publication No. 50-118294(JP-A-118284/75) or in "Fine Ceramic Handbook" by K. Yano, published byASAKURA SHOTEN in 1984; (b) a material mainly composed of a compoundhaving a spinel type crystal structure, as disclosed for example in JPPatent KOKAI Publication No. 48-63985 (JP-A-63995/74), (c) a materialmainly composed of zirconia, as disclosed in, e.g., "Nainenkikan"(Internal Combustion Engine) vol 30, No. 8, page 98, and (d) a materialmainly composed of a compound having a perovskite type crystalstructure.

SUMMARY OF THE DISCLOSURE Problems to be Solved by the Invention

However, according to the eager investigation of the inventors of thepresent invention the following problems have been turned out. With thematerial (a) mainly composed of the corundum crystal structure, theresistance--temperature characteristics can not be adjusted to a largerextent. If, for example, the additive is added in an excess quantity,the structure ceases to remain the stable corundum type crystalstructure, resulting in deteriorated thermal stability.

The material (b) mainly composed of the spinel type crystal structurehas a higher rate of change of the resistance versus temperature (ahigher temperature gradient constant β), so that it cannot be employedover a wider temperature range. On the other hand, materials mainlycomposed of NiAl₂ O₄ or CoAl₂ O₄ are low in the thermal resistance andhence cannot be employed at elevated temperatures.

The material mainly composed of zirconia (c) is oxygen ion conductiveand is increased in resistance in a temperature range lower than theactivation temperature so that it cannot be employed practically.

The material mainly composed of the compound having the perovskite typecrystal structure (d) has such inconvenience that, if only a slightamount of La oxides remain unreacted, the non-reacted La componentreacts with the atmospheric moisture to form unstable La(OH)₂ with theresult that the device composed of the material collapses or exhibitsonly unstable resistance values.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a improved novelceramic composition for thermistor.

It is another object of the present invention to provide a ceramicthermistor which overcomes the above problems.

It is a further object of the present invention to provide a ceramiccomposition for thermistor which has a broad range of resistance valuesby adjusting the composition of the material.

It is a still further object of the present invention to provide aceramic composition for thermistor which can be sintered at atemperature not higher than 1600° C. to prevent electrode deterioration,which is free from hygroscopic substances and the resulting sinteredbody is less susceptible to deterioration in characteristics due to theatmospheric humidity or heat hysteresis and which can be employed over awide temperature range of from room temperature to 1100° C.

It is a further object of the present invention to provide not only theceramic compositions aforementioned, but to provide an improvedthermistor elements which are obtainable by sintering the aforesaidcompositions.

It is also an object of the present invention to provide a process forproducing the aforementioned compositions and thermistor element usingthe same.

Still further objects of the invention will become apparent from theentire disclosure.

At least one of the said objects is accomplished by the followingceramic composition and a thermistor element obtainable from the samecomposition. The present invention further provides a process forproducing a thermistor element.

The ceramic composition for the thermistor according to the presentinvention is represented by the formula (M¹ M² O₃)_(1-x) ·(M¹ AlO₃)_(x),where

M¹ is one or more elements selected from the elements belonging to thegroup 3A excluding La, M² is one or more elements selected from theelements belonging to the groups 4A, 5A, 6A, 7A and 8 and 0.8>×>0.

Part of the elements may be inter-diffused each other, such as the casewith certain element(s) of M² and Al.

Definition

The groups 3A, 4A, 5A, 6A and 7A herein mean 3A, 4A, 5A, 6A and 7A ofthe Periodic Table according to the agreement in 1965 of the Committeefor the Nomenclature for Inorganic Chemistry of International Union ofPure and Applied Chemistry (IUPAC).

According to the process aspect of the present invention, there isprovided a process for producing a composition for thermistorcomprising:

providing M¹ M² O₃ and M¹ AlO₃, respectively, where M¹ is one or moreelements selected from the elements belonging to the group 3A excludingLa, M² is one or more elements selected from the elements belonging tothe groups 4A, 5A, 6A, 7A and 8,

then providing a mixture comprising (M¹ M² O₃)_(1-x) ·(M¹ AlO₃)_(x)where 0.8>×>0.

PREFERRED EMBODIMENTS

The ceramic composition for the thermistor represented by (M¹ M²O₃)_(1-x) ·(M¹ AlO₃)_(x), where M¹ is one or more elements selected fromthe elements Y, Sm, Pr, Nd, Dy, Ho, Er, Gd and Yb and M² is one or moreelements selected from the elements Cr, Ti, Mn, Fe and Co, where0.8>×>0, does not exhibit hygroscopicity and is free from deteriorationin strength and superior in thermal resistance. Preferred of x is 0.01or more for effective role of x.

The ceramic composition for the thermistor with x being 0.05 to 0.5 doesnot exhibit hygroscopicity and is free from deterioration in strengthand superior in thermal resistance, so that it can be adjusted topractically satisfactory resistance values. More preferred of x is0.1-0.4.

The ceramic composition for the thermistor mainly composed of acomposition comprising (M¹ M² O₃)_(1-x) ·(M¹ AlO₃)_(x) admixed with asintering aid (or aids) for improving sinterability exhibits highstrength and superior thermal resistance. Any sintering aid capable offorming a liquid phase in the grain boundary for improving sinterabilityof the ceramics suffices. Silica, mullite or the like is preferred. Theamount of addition of the sintering aid is 0.5 to 10 wt % and preferablyranges between 0.8 to 5 wt %.

M¹ M² O₃ is a substance exhibiting higher electrical conductivity and M¹AlO₃ is a substance exhibiting lower electrical conductivity. Athermistor may be provided in which, by changing the mixing ratio ofthese two substances, the resistance values can be easily changed andmay be maintained stably at higher temperatures. The reason thethermistor exhibits superior stability at elavated temperatures ispresumably that such substances as M¹ M² O₃ or M¹ AlO₃ are stable atelevated temperatures, and that the melting points of YCrO₃ and YAlO₃compounds, for example, are as high as approximately 2300° C. and 1900°C., respectively.

It has been found that a mixed system of YCrO₃ having high electricalconductivity and YAlO₃ having high electrical insulating properties isfree from yielding of subsidiary components on sintering and, aftersintering, is mainly composed of YCrO₃ and YAlO₃ phases, if a matrixcomposed of the sintering aids or unavoidable impurities aredisregarded. It has also been found that the mixed system exhibitssimple reactions and high stability and may be easily adjusted inresistance, such that it can be employed over a broad temperature rangeof from 300° C. to 1100° C.

According to the process aspect, each of the calcined masses M¹ M² O₂and M¹ AlO₃ is pulverized to form a fine powder, preferably having amean particle size of about 1 μm or less. The resulting fine powders aremixed together in a desired proportion, without or preferably with asintering aid.

The resultant powdery mixture is formed, e.g., by press-forming,molding, or other known shaping method followed by sintering.

Each calcination for M¹ M² O₃ or M¹ AlO₃ is carried out by pre-firingeach of a starting raw material mixture with a suitable proportion underthe condition that yields synthesized M¹ M² O₃ or M¹ AlO₃, namely at asufficient temperature for forming calcined/synthesized mass of eachcompound. The calcination serves to a better sinterability at arelatively low temperature resulting in a homogeneous product withouthetrogeneous reaction grades.

The calcining temperature generally ranges from 1200° to 1400° C. sothat unreacted residue is minimized and sintering does not proceed inexcess so as not to offer difficulty in pulverization.

The sintering is carried out under the conditions that will sinter themass-to-be sintered to a sufficient relative density, e.g., 90%, 95% or98% higher of the theoretical. The sintering temperature does not muchdepend on the selected elements for M¹, M², and generally lies between1450°-1600° C., preferably about 1550° C. Rather the sinteringtemperature will be affected by the kind and amount of the sinteringaids. The sintering temperature is selected so as to give high stabilityin the electric properties and a sufficient density of the sinteredproducts.

According to the preferred embodiments of the present invention, it isgenerally possible to adjust the resistance values within a desiredrange according to the proportion of the ingredients (mixing ratios),yet more, with a reduced, controlled resistance change rate in a widertemperature range of 300° to 900° C.

According to the present invention, a ceramic composition for athermistor could be produced which is superior in stability at elevatedtemperature and mechanical strength, and which can be used over a broadtemperature range such that it can be used in, for example, aoverheating detection device for a catalyst used for cleaning theexhaust gases of the automotive vehicles, a hot gas temperature sensor,such as a sensor for recirculated gases in an exhaust gas circulatingdevice, a high temperature measurement device for an area exposed to ahigher temperature or a temperature sensor for a variety of furnaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the present invention.

Explanation of Numerals

1--thermistor element; 2--electrode.

EXAMPLES

In the following the present invention will be further elucidated withreference to the preferred embodiments which are not restrictive.

EXAMPLE 1

Example 1 of the present invention is now explained.

Y₂ O₃ having a purity of 99.9% or higher and a mean particle size of 1μm and Cr₂ O₃ having a purity of 98.5% or higher and a mean particlesize of 1 μm were weighted to a molar ratio of 1:1, mixed by a wetmixing method, dried and calcined (pre-fired) by subsequentlymaintaining the mixture 1400° C. for two hours. The resulting calcinedYCrO₃ exhibiting high electrical conductivity was pulverized to givepowders having a mean particle size of approximately 1 μm.

On the other hand, Y₂ O₃ having a purity of 99.9% or higher and a meanparticle size of 1 μm and Al₂ O₃ having a purity of 99.9% or higher anda mean particle size of 1 μm were weighted to a molar ratio of 1:1,mixed by a wet mixing method, dried and calcined (pre-fired) bysubsequently maintaining the mixture at 1400° C. for two hours. Theresulting calcined YAlO₃ exhibiting high electrical insulatingproperties was pulverized to give powders having a mean particle size ofapproximately 1 μm.

YCrO₃ and YAlO₃ thus produced were weighed from sample to sample atmixing ratios shown in Table 1 and admixed with 1 wt % of SiO₂ powdershaving a mean particle size of 0.6 μm. The resulting mass was mixedtogether by a wet method (ball mill) to give a slurry mixture which asthen passed through a 60-mesh sieve and subsequently dried. The driedmass was then admixed with a binder composed of 15 wt % of PVB, 10 wt %of DBP, 50 wt % of MEK and 25 wt % of toluene for granulating powdersfor press molding.

                                      TABLE 1                                     __________________________________________________________________________                resistance value       resistance                                 Sample                                                                            composition                                                                           (KΩ)   β    change rate (%)                            Nos.                                                                              YCrO.sub.3                                                                        YAlO.sub.3                                                                        300° C.                                                                     650° C.                                                                    900° C.                                                                    300-650                                                                            650-900                                                                            300°                                                                        900° C.                        __________________________________________________________________________     1* 100  0  0.45 0.085                                                                             0.06                                                                              2520 1510  15[-18]                                                                           10[-81]                               2   95   5  13.2 0.382                                                                             0.130                                                                             5350 4670 15[-8]                                                                             10[-27]                               3   90  10  35.6 0.780                                                                             0.240                                                                             5770 5104 12[-6]                                                                             9[-23]                                4   80  20  70.0 0.905                                                                             0.285                                                                             6570 5000  8[-4]                                                                             6[-16]                                5   70  30  96.4 1.081                                                                             0.327                                                                             6790 5180  7[-3]                                                                             5[-13]                                6   65  35  157  1.824                                                                             0.478                                                                             6730 5800 10[-5]                                                                             8[-17]                                7   60  40  296  2.836                                                                             0.735                                                                             7020 5850 13[-6]                                                                             7[-16]                                8   55  45  363  3.591                                                                             0.952                                                                             6980 5750 15[-7]                                                                             9[-20]                                9   50  50  497  5.100                                                                             1.385                                                                             6920 5650 15[-7]                                                                             8[-18]                                10* 20  80  >30000                                                                             57.0                                                                              10.3                                                                              --   7409 --   --                                    __________________________________________________________________________     *marks indicate Comparative Examples                                     

The resulting powders were charged into a metal mold comprised of twoplatinum wires, each 0.4 mm in diameter, placed at an interval of 1.2mm, and were pressed under a pressure of 98 MPa (1000 kg/cm²), forproducing a molded product, 3 mm in diameter and 2 mm in thickness,having two electrode lines of platinum wires, as shown in FIG. 1. Themolded product was sintered in ambient air at 1550° C. to produce athermistor element.

On the thermistor element, thus produced, resistance values in theatmosphere at 300° C., 650° C. and 900° C. were measured, and values ofthe temperature gradient constant β were calculated. The results areshown in the columns of the resistance and β. Next, the samples weremaintained for 300 hours in the atmosphere at 1000° C. and theresistance values thereof at 300° C., 650° C. and 900° C. before andafter the maintenance at 1000° C. were measured in order to check thedurability. The results are shown in the resistance change rate columnin Table 1. The inventive article exhibits high strength as lead wiresand can be easily built into temperature sensors. This may presumably beascribable to the lower sintering temperature.

In Table 1, β indicates the temperature gradient constant. Thetemperature gradient constant β, the resistance change rate and theresistance change rate calculated as temperature (converted temperaturevalue derived from the resistance change rate) are defined by thefollowing equation:

    β=ln(R/R.sub.o)/(1/K-1/K.sub.o)

    resistance change rate=(R.sub.t -R.sub.o)/R.sub.o ×100%

where ln indicates common logarithm, and R and R_(o) indicate resistancevalues at absolute temperatures K and K_(o), respectively. In Table 1,300-650 and 650-900 denote the temperature gradient constants β between300° C. and 650° C. and between 650° C. and 900° C., respectively.

R_(t) denotes the resistance value at an absolute temperature K_(t)(t=300° C. or 900° C.) after maintenance at 1000° C. for 300 hours.

The values in the resistance change rate column, shown within squirebrackets [ ], denote resistance changes before and after the test ondurability, as converted into temperatures, and are defined by thefollowing equation:

    (Resistance change rate, calculated as temperature)=β×K.sub.o /(ln(R.sub.t /R.sub.o)×K.sub.o +β)-K.sub.o

It is seen from Table 1 that, by changing the YCrO₃ /YAlO₃ mixing ratio,the resistance value can be adjusted easily. It is contemplated that noauxiliary components are yielded in the reaction between YCrO₃ and YAlO₃and a two-phased mixture is produced after sintering, with the reactionsystem being simple, so that the resistance value can be adjusted veryeasily.

However, if the YAlO₃ ratio exceeds 80%, the sinterability is lowered,while the resistance value at 300° C. exceeds 30 megohms(MΩ) and itbecomes difficult to measure the resistance value at a temperature rangebelow 300° C., such that the thermistor becomes unsuitable for sensinglower temperatures. Above all, if the resistance value of the thermistorelement is set so as to be higher than the insulation resistance betweenharness lead wires, the thermister element can hardly be used fordetecting the temperature of the exhaust gases of automotive vehicles oras an alarm device for preventing overheating of the catalytic devicefor cleaning the exhaust gases of the automotive vehicles. Theinsulation resistances between the lead wires is occasionally lowered toa level of tens of megohms(MΩ). The sample number 10 has an insulatingresistance of 30 megohms or higher and hence becomes unusable.

The resistance change rate of the thermistor element, as measured by thetest on durability, was less than 20%, with an exception of samplenumber 10. Sample number 1 can hardly be used as a thermistor elementfor a detector device because the value of the constant β thereof islow, and equal to -81° C. in terms of the converted temperature value,thus exhibiting poor temperature accuracy.

Sample number 7 was maintained for ten hours in the atmosphere at 1100°C. and resistance values thereof at 300° C. and 900° C. before and aftersuch maintenance were measured in order to check the resistance changerate. It was found that the change rate was 7% at 300° C., thusindicating the change of -3° C. in terms of the converted temperaturevalue. The resistance change rate was 12% at 900° C., thus indicatingthe change of -26° C. in terms of the converted temperature value.

EXAMPLE 2

Example 2 is now explained.

Yb₂ O₃ having a purity of 99.9% or higher and a mean particle size of1.5 μm and Cr₂ O₃ having a purity of 98.5% or higher and a mean particlesize of 1 μm were weighed to a molar ratio of 1:1, mixed together by awet mixing method, dried and subsequently calcined maintaining themixture at 1400° C. for two hours. The resulting calcined YbCrO₃,exhibiting high electrical conductivity, was pulverized to give powdershaving a mean particle size of approximately 1 μm.

On the other hand, Yb₂ O₃ having a purity of 99.9% or higher and a meanparticle size of 1.5 μm and Al₂ O₃ having a purity of 99.9% or higherand a mean particle size of 1 μm were weighed to give a molar ratio of1:1, mixed together by a wet mixing method, dried and subsequentlycalcined maintaining the mixture at 1400° C. for two hours. The calcinedmass was pulverized to give powders having a mean particle size ofapproximately 1 μm.

The two kinds of the powders, produced as described above, were weighedto give a composition shown in Table 2, and a thermistor element wasproduced by the same method as that of Example 1, and the variousproperties thereof were measured. The results are shown in Table 2. Bythe way, the respective columns of Tables 2 to 4 have the same meaningas Table 1.

Sample number 11, represented by Yb(Cr₀.60 Al₀.40)O₃, has the resistanceof 150 kiloohms(KΩ) at 300° C., such that a thermistor element isproduced which is capable of being employed in a temperature range fromlower temperatures up to higher temperatures.

                                      TABLE 2                                     __________________________________________________________________________    Sample                                                                            composition  resistance value (KΩ)                                                               β    resistance change rate (%)             Nos.                                                                              YbCrO.sub.3                                                                        Yb.sub.2 O.sub.3 --Al.sub.2 O.sub.3                                                   300° C.                                                                    650° C.                                                                    900° C.                                                                    300-650                                                                            650-900                                                                              300°                                                                       900° C.                   __________________________________________________________________________    11  60   40      150 1.467                                                                             0.450                                                                             6990 5120   16[-7]                                                                            6[-15]                           __________________________________________________________________________

The sample number 11 was maintained for ten hours in atmosphere at 1100°C. and the resistance values thereof at 300° C. and 900° C. before andafter such maintenance were measured in order to check the resistancechange rate. It was found that the change rate at 300° C. was 11% whichcorresponds to a change of -5° C. in terms of the converted temperaturevalue. The change rate at 900° C. was 7%, which corresponds to a changeof -17° C. in terms of the converted temperature value.

EXAMPLE 3

Example 3 will be now explained. A thermistor element having acomposition shown in Table 3 was produced by the same method as inExample 1 except using Er₂ O₃ having a purity of 99.9% and a meanparticle size of 1 μm. Measurements similar to those in Example 1 weremade of the thermistor elements, and the results shown in Table 3 wereobtained.

                                      TABLE 3                                     __________________________________________________________________________    Sample                                                                            composition                                                                             resistance value (KΩ)                                                               β    resistance change rate (%)                Nos.                                                                              ErCrO.sub.3                                                                        ErAlO.sub.3                                                                        300° C.                                                                    650° C.                                                                    900° C.                                                                    300-650                                                                            650-900                                                                              300°                                                                       900° C.                      __________________________________________________________________________    12  60   40   250 2.010                                                                             0.590                                                                             7288 5308   12[-5]                                                                            7[-18]                              __________________________________________________________________________

Sample number 12 was maintained for ten hours in the atmosphere at 1100°C. and resistance values at 300° C. and 900° C. before and after suchmaintenance were measured in order to check the resistance change rate.The change rate at 300° C. was 14% which was the change of -6° C. interms of the temperature. The change rate at 900° C. was 8% whichcorresponded to the change of -20° C. in terms of the temperature.

Example 4 is now explained. Thermistor elements having the compositionsshown in Table 4 were produced by the same method as in Example 1 exceptusing Gd₂ O₃ having a purity of 99.9% and a mean particle size of 1 μmin place of Y₂ O₃. Measurements were made of these elements in the sameway as in Example 1, and the results shown in Table 4 were obtained.

In Table 4, the resistance change rate denotes the resistance changerate before and after the elements were maintained for 300 hours at1000° C. and the corresponding change in terms of the convertedtemperature values. It is seen that, by setting X in the composition of(GdCrO₃)_(1-x) ·(YAlO₃)_(x) so as to be 0.2 to 0.5, the thermistorelements could be produced which exhibited practically optimumresistance values and which exhibited only small resistance change ratevalue and the resistance change value in terms of the convertedtemperature value even when the elements were maintained at highertemperatures.

                                      TABLE 4                                     __________________________________________________________________________                 resistance value      resistance                                 Sample                                                                            composition                                                                            (KΩ)  β    change rate (%)                            Nos.                                                                              GdCrO3                                                                             YAlO.sub.3                                                                        300° C.                                                                    650° C.                                                                    900° C.                                                                    300-650                                                                            650-900                                                                            300°                                                                         900° C.                       __________________________________________________________________________    13  80   20  450 0.530                                                                             0.165                                                                             6710 5050 15.6[-7]                                                                            3.8[-10]                             14  60   40  150 1.650                                                                             0.430                                                                             6820 5820 18.3[-8]                                                                            4.8[-11]                             15  50   50  212 2.950                                                                             0.75                                                                              6460 5930 22.2[-10]                                                                           4.0[-9]                              __________________________________________________________________________

Sample number 14 was maintained for ten hours in the atmosphere at 1100°C. and the resistance values at 300° C. and 900° C. before and aftersuch maintenance were measured in order to check the resistance changerate. It was found that the change rate at 300° C. was 18.4%, whichcorresponded to the change of -8° C. in terms of the convertedtemperature value. The resistance change rate at 900° C. was 44%, whichcorresponded to the change of -10° C. in terms of the convertedtemperature value.

Besides the liquid phase forming materials which are essential, thesintering aids may further optionally comprise one or more of thefollowing materials CaO--SiO₂ type compounds, SrO--SiO₂ type compounds,MgO--SiO₂ type compounds, B₂ O₃ --SiO₂ --Al₂ O₃ type compounds and B₂ O₃--SiO₂ type compounds.

It should be noted that modification obvious for one skilled in the artcan be done without departing from the essential gist and scope of thepresent invention as disclosed hereinabove and claimed hereinbelow.

What is claimed is:
 1. A ceramic composition for thermistor representedby the formula (M¹ M² O₃)_(1-x) ·(M¹ AlO₃)_(x), whereM¹ is one or moreelements selected from the elements belonging to the group 3A of theInternational Periodic Table excluding La, M² is one or more elementsselected from the elements belonging to the groups 4A, 5A, 6A, 7A, and 8of the International Periodic Table and 0.8>×>0, wherein a sintering aidis added to the composition.
 2. The ceramic composition for thermistoras defined in claim 1 wherein M¹ is one or more elements selected fromthe group consisting of Y, Sm, Pr, Nd, Dy, Ho, Er, Gd and Yb, and M² isone or more elements selected from the group consisting of Cr, Ti, Mn,V, Fe and Co.
 3. The ceramic composition for thermistor as defined inclaim 1 or 2 wherein x is 0.05 to 0.5.
 4. The ceramic composition forthermistor as defined in claim 1 wherein M¹ is Y or Gd and M² is Cr. 5.The ceramic composition for thermistor as defined in claim 3 wherein M¹is Y or Gd and M² is Cr.
 6. The ceramic composition for thermistor asdefined in claims 2 or 4 wherein the sintering aid comprises silicaand/or mullite.
 7. The ceramic composition for thermistor as defined inclaim 3 wherein the sintering aid comprises silica.
 8. The ceramiccomposition for thermistor as defined in claim 1 wherein the sinteringaid is present in an amount of 0.5 to 10 wt %.
 9. The ceramiccomposition for thermistor as defined in claim 1 wherein the sinteringaid is present in an amount of 0.8 to 5 wt %.
 10. The ceramiccomposition for thermistor as defined in claim 1 wherein the sinteringaid comprises silica and/or mullite.
 11. The ceramic composition forthermistor as defined in claim 7 wherein the sintering aid furthercomprises at least one member selected from the group consisting ofCaO--SiO₂ compounds, SrO--SiO₂ compounds, MgO--SiO₂ compounds, B₂ O₃--SiO₂ --Al₂ O₃ compounds and B₂ O₃ --SiO₂ compounds.
 12. A thermistorelement having the ceramic composition as defined in any one of claims1, 2 or 4 as a main constituent.
 13. A thermistor element having theceramic composition as defined in claim 3 as a main constituent.
 14. Athermistor element having the ceramic composition as defined in claim 7as a main constituent.
 15. A process for producing a composition forthermistor comprising:providing M¹ M² O₃ and M¹ AlO₃, respectively,where M¹ is one or more elements selected from the elements belonging tothe group 3A of the International Periodic Table excluding La, M² is oneor more elements selected from the elements belonging to the groups 4A,5A, 6A, 7A, and 8 of the International Periodic Table, providing amixture comprising (M¹ M² O₃)_(1-x) ·(M¹ AlO₃)_(x) where 0.8>×>0, andadding a sintering aid to the mixture.
 16. The process as defined inclaim 15, wherein each of said M¹ M² O₃ and M¹ AlO₃ is produced throughcalcinating a starting raw material mixture to form a calcined mass. 17.The process as defined in claim 15 wherein M¹ is one or more elementsselected from the group consisting of Y, Sm, Pr, Nd, Dy, Ho, Er, Gd andYb and M² is one or more elements selected from the group consisting ofCr, Ti, Mn, V, Fe and Co.
 18. The process as defined in claim 15 whereinx is 0.05 to 0.5
 19. The process as defined in claim 15 or 18 wherein M¹is Y or Gd and M² is Cr.
 20. The process as defined in any one of claims15, 16 or 17 wherein the sintering aid comprises silica and/or mullite.21. The process as defined in claim 10, wherein each of said calcinedmasses is pulverized to form a fine powder.
 22. A process for producinga thermistor element, comprising the process as defined in claim 21, andfurther comprising sintering said pulverized powders.
 23. The process asdefined in claim 22, wherein the sintering aid is admixed to saidpulverized powders.
 24. The process as defined in claim 19 wherein thecalcination is carried out approximately at 1200°-1400° C.
 25. Theprocess as defined in claim 19 wherein the calcination is carried outapproximately at 1400° C.
 26. A process for producing a thermistorelement, comprising the process as defined in claim 24, and furthercomprising sintering said pulverized powders.
 27. The process as definedin claim 22 or 23 wherein the sintering is carried out at a temperatureapproximately of 1450°-1600° C.
 28. The process as defined in claim 22or 23 wherein the sintering is carried out at a temperatureapproximately of 1550° C.
 29. The process as defined in claim 20 whereinthe sintering aid further comprises at least one compound selected fromthe group consisting of CaO--SiO₂ compounds, SrO--SiO₂ compounds,MgO--SiO₂ compounds, B₂ O₃ --SiO₂ --Al₂ O₃ compounds and B₂ O₃ --SiO₃compounds.